1. Field of the Invention
This invention is related to image forming apparatuses and process cartridges.
2. Description of the Related Art
The electrophotography is used in electrophotographic apparatuses such as copy machines and laser beam printers since the electrophotography enables high-speed, high-quality printing.
In general, the photoreceptors used in the electrophotographic apparatuses have been organic photoreceptors containing photoconductive organic materials, recently. Regarding the constitutions of the photoreceptors, separated function photoreceptors have been used in which a charge generating material and a charge transfer material are dispersed in different layers (charge generating layer and charge transfer layer).
In recent years, office functions have been improved in quality and in speed; there have been demands for faster and colorized document processing and faster, colorized, high-quality image forming apparatuses (copying machines, printers, facsimile machines, etc.) for processing the documents. In order to satisfy the demands, tandem-type color image forming apparatuses have been proposed and commercialized. A tandem-type color image forming apparatus has separate image forming units for respective colors of black (K), yellow (Y), magenta (M), and cyan (C). The tandem-type color image forming apparatus forms images of different colors in the respective image forming units, then transfers the images onto a transfer member or an intermediate transfer member so that the images overlap, thus forming a color image.
In the color image forming apparatuses, techniques have been proposed (for example in Japanese Patent Application Laid-Open (JP-A) No. 2003-241551) which can switch between different image formation modes in accordance with the image and the kind of the image receiving medium, thus realizing high quality and high efficiency. For example, only the black toner is used when black-and-white image is formed, whereby the process speed can be supposedly higher than in the color image formation. A high-quality image can supposedly be formed by lengthening the time required for the image formation when the image receiving medium is heavy paper or an OHP sheet, in both cases of color image forming apparatuses and monochromatic image forming apparatuses.
However, if the time required for the processes from charging to development varies as in the above techniques, the image quality tends to be insufficient. When the image formation mode is switched to another image formation mode, the time required for the processes from charging to development changes, but photoreceptors sufficiently adaptable to the change in the use condition have not been developed. Therefore, in such techniques, there have been a problem that, when image formation is conducted in a process condition in which a longer time is required for the processes from charging to development, fogging and black spots develop severely and image memory easily occurs.